This invention is concerned with magnetic markers that are used in the detection of hidden elongated objects, such as buried plastic utility pipes, ducts or conduits, or fiber optic cables. The invention is concerned, more particularly, with elongated helical or twisted magnetic markers, i.e., convoluted magnetic markers, and with the manufacture of such magnetic markers and others.
The aforesaid co-pending applications disclose and claim new and improved methods and apparatus for marking, locating, tracing, and identifying hidden elongated objects, such as buried, non-conductive cables (e.g., fiber optic cables) and non-conductive pipes, tubes, ducts, and conduits. In one form of the invention disclosed and claimed in the co-pending applications, a magnetic marker comprises an elongated permanent magnet device that is provided on an elongated hidden object to be detected, with the length of the device extending along the length of the object. The device comprises a helical strip magnetized in the direction of its width. With such a device the orientation of the magnetic axis varies at different positions along the length of the object to provide a magnetic field signature for locating, tracing, and identifying the object.